OneTuple fills the "tuple gap" with a singleton tuple.  It does
not support the usual parenthesized tuple syntax (which would be
"(a)" instead of "OneTuple a"). It
* has the expected laziness properties,
* can be pattern-matched,
* ships with instances for several standard type classes,
  including all those supported by H98-standard tuples,
* requires no language extensions, except for hierarchical modules.
